The present disclosure relates to printable media transport and hold-down systems. More specifically, the present disclosure relates to movable guides used to overlap and hold down printable media such as paper, cardstock, or other substrates.
Direct-to-paper ink jet printing systems typically include a printable media hold-down system. As a printable medium passes on a transport surface under an ink jet print head, the hold-down system attempts to prevent contact between the printable medium and the print head. Contact between printable media and the print head may result in fibers from printable media becoming lodged in ink nozzles in the print head. Over time, a substantial number of fibers could become lodged in the nozzles causing the print head to clog. A clogged print head can damage printable media by printing incorrectly, wasting ink and causing significant downtime as the clogged head must be cleaned and/or replaced.
Some high speed printing systems, or systems for printing larger sizes of printable media, may require a large array of print heads. A clogged print head is especially troubling when using a print head array. Cleaning and/or replacing the print heads in a print head array can cause an even greater downtime depending on the size of the print head array.
Several hold-down systems are prevalent in modern direct-to-paper printing systems. One example is a vacuum/plenum system. In this system, a series of small holes are placed in the transport surface, and air is sucked through the holes, away from the print head (or print head array). As the printable medium passes under the print head (or print head array), a vacuum is created under the printable medium, thereby holding the printable medium against the transport surface.
Another exemplary hold-down system is an electrostatic tacking hold-down system. In this system, the transport surface is electrostatically charged, resulting in the printable medium tacking, or electrostatically sticking, to the transport surface as the printable medium moves under the print head (or print head array).
Both of these hold-down systems have inherent problems, however. Specifically, both of these approaches have limits to the amount of force that can be applied across printable media to protect printable media from coming into contact with the print head (or print head array). Both of these approaches are particularly susceptible to failure at the corners and edges of printable media. At the corners and edges, the downward force caused by the vacuum is less than at other portions of a printable medium due to air leakage around the edge of the printable medium, and the force exerted by an electrostatic system decreases if the sheet edge is not in intimate contact with the belt. Also, at the corners at edges, the bending moment imparted by the vacuum or the electrostatic tacking is lowest, which can result in the corners and edges bending away from the transport surface and contacting the print head (or print head array).